ICD-10 Code F06.31: Mood Disorder Due to Known Physiological Condition

Key Takeaways

F06.31 is a billable ICD-10-CM code for mood disorder due to a known physiological condition, with depressive features, valid for FY2026.

A code for the underlying physiological condition (such as G20 for Parkinson’s or E03.9 for hypothyroidism) must be sequenced first before F06.31.

F06.31 requires documented clinician causality – the chart must establish that the depressive features are directly caused by the physiological condition, not a coincidental primary mood disorder.

Pabau’s mental health EMR supports structured clinical documentation and streamlined claims workflows for codes like F06.31.

ICD-10 Code F06.31: Definition and Clinical Description

Most denial letters for secondary mood disorders don’t cite a missing diagnosis. They cite missing causality. ICD-10 Code F06.31 – Mood disorder due to known physiological condition, with depressive features – requires clinicians to document not just that depression is present, but that a specific medical condition is directly responsible for it. That distinction separates F06.31 from every other depressive code in the ICD-10-CM system, and it’s where most coding errors originate. Practices using a mental health EMR with structured documentation fields are significantly better positioned to capture this relationship accurately at the point of care.

F06.31 sits within the ICD-10-CM hierarchy as follows: category F06 (Other mental disorders due to known physiological condition) contains subcategory F06.3 (Mood disorder due to known physiological condition), which breaks into four billable specificity codes. F06.31 captures the depressive features variant, distinguishing it from F06.32 (manic features), F06.33 (mixed features), and F06.34 (anxiety features). The CDC/NCHS ICD-10-CM web tool confirms F06.31 is a valid, billable code effective for FY2026.

In DSM-5 terms, F06.31 corresponds most closely to Depressive Disorder Due to Another Medical Condition. The mapping is widely referenced in clinical crosswalk tables, though practitioners should note the ICD-10-CM and DSM-5 systems are not identical in scope or criteria. The APA’s crosswalk documentation should be consulted when translating DSM-5 diagnoses to ICD-10-CM for billing purposes.

Common Physiological Conditions That Trigger F06.31

The physiological condition must be documentable and clinically established. Several medical diagnoses are commonly associated with secondary depressive mood disorders:

• G20 (Parkinson’s disease) – Dopaminergic depletion frequently presents with clinically significant depressive episodes.
• G35 (Multiple sclerosis) – Neuroinflammatory changes and psychosocial burden both contribute to depressive features.
• E03.9 (Hypothyroidism, unspecified) – Thyroid hormone deficiency has a well-established causal relationship with depressive symptoms.
• C80.1 (Malignant neoplasm) – Cancer-related depression meeting clinical criteria may support F06.31 when causality is documented.
• Traumatic brain injury, Cushing’s syndrome, and post-stroke conditions are also commonly cited etiologies.

The physiological condition code is always sequenced first in the claim. F06.31 follows as a secondary diagnosis. This sequencing rule is non-negotiable under ICD-10-CM Official Guidelines.

Coding Guidelines and Sequencing Rules

Sequencing errors are the fastest route to a denied claim. Under ICD-10-CM guidelines for “due to” etiology codes, the underlying physiological condition must appear as the principal or first-listed diagnosis. F06.31 follows as an additional code. Reversing this order – listing F06.31 first – misstates the clinical picture and triggers edits in most payer systems. Practices using psychiatry EMR software with built-in coding logic can flag this sequencing requirement automatically during documentation.

The clinician – not the coder – must establish the causal relationship in the clinical documentation. This is a physician attestation requirement. Coders cannot infer causality from a list of diagnoses; they need an explicit clinical statement such as “depressive disorder secondary to hypothyroidism” or “mood disturbance attributable to Parkinson’s disease.” Without this, the code cannot be applied even if both diagnoses appear in the record.

Related Codes Within F06.3

F06.31 is one of four specificity codes under F06.3. Selecting the correct sibling code depends on the documented mood features:

• F06.31 – Depressive features (persistent low mood, anhedonia, fatigue caused by the physiological condition)
• F06.32 – Manic features (elevated, expansive, or irritable mood caused by the physiological condition)
• F06.33 – Mixed features (concurrent depressive and manic features)
• F06.34 – Anxiety features (prominent anxiety caused by the physiological condition)

Billing the parent code F06.3 without the fourth digit (the specific feature type) is not billable. F06.3 is a non-billable subcategory. F06.31 through F06.34 are the valid billable codes. Per the CMS ICD-10 codes page, only the most specific code level should be assigned.

F06.31 vs. F32.9: Choosing the Right Code

This is the differential that trips up the most practices. Both F06.31 and F32.9 describe depressive presentations, but they represent fundamentally different clinical situations. Getting this wrong can result in a denial, a compliance audit finding, or – more seriously – a misrepresentation of the patient’s clinical picture. For background on how related mood and anxiety codes intersect in ICD-10-CM, see our anxiety ICD-10 code reference.

A practical test: if removing the physiological condition from the clinical picture would eliminate the depressive presentation, F06.31 is appropriate. If the patient would likely have depression regardless of the medical condition, F32.9 or another primary depressive code applies. This distinction must be in the clinical note, not inferred from the diagnosis list.

Documentation Requirements for F06.31

Strong documentation is what keeps F06.31 claims paid and audit-proof. The chart must contain three core elements for this code to be defensible. Practices that have moved to digital intake forms and structured clinical notes are better positioned to capture all three consistently across providers.

1. Named physiological condition – The specific medical diagnosis causing the mood disturbance must be named. “Medical illness” is insufficient; “Parkinson’s disease (G20)” is acceptable.
2. Explicit causal statement – The clinician must document the causal link in their own words. Acceptable language includes: “depression secondary to,” “mood disturbance attributable to,” “depressive features caused by.”
3. Depressive features described – The clinical note must describe the specific depressive features present (e.g., persistent low mood, anhedonia, psychomotor retardation, sleep disturbance). Vague references to “depression” are insufficient.

AHA Coding Clinic guidance reinforces that physician documentation is the foundation for all etiology-based codes. When documentation is ambiguous, the AHA recommends querying the physician rather than coding based on inference. This query should be documented in the medical record.

HIPAA and Compliance Considerations

Under HIPAA’s code set requirements, covered entities must use ICD-10-CM codes at their highest level of specificity for electronic transactions. Billing F06.3 (non-billable) instead of F06.31 violates this requirement and will be rejected by clearinghouses. The specificity requirement is not optional – it is a HIPAA compliance matter, not just a billing preference.

Billing and Coverage Considerations

F06.31 is billable under Medicare and Medicaid when medical necessity is established and documentation meets payer requirements. Coverage is not automatic, and some payers apply additional criteria. Practices managing high volumes of these claims benefit from claims management software that can flag sequencing errors and missing documentation fields before submission.

• Medicare: Covered when medically necessary. The claim must demonstrate that the mental disorder is directly related to the documented physiological condition. Medical necessity must be supported in the clinical record.
• Medicaid: Coverage varies by state. Some state Medicaid programs require prior authorization for mental health services billed alongside complex medical diagnoses. Verify payer-specific policies before submission.
• Commercial payers: Policies vary. Some payers require specialist attestation (e.g., neurologist or internist) when the physiological condition is complex. Review LCDs and payer-specific coding policies for codes in the F06 family.
• Prior authorization: Mental health services for patients with complex co-occurring medical conditions may trigger PA requirements. Check payer portals before scheduling ongoing treatment.

The AAPC ICD-10-CM lookup tool allows coders to verify F06.31 against current code descriptors, inclusion notes, and coding guidelines. Cross-referencing with the official tabular list before submitting claims reduces the risk of submission errors.

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Related and Excluded Codes

Understanding which codes are adjacent to F06.31 prevents undercoding, overcoding, and inappropriate code substitution. The F06 category covers several distinct mental disorder presentations, and each has specific clinical criteria. For a broader reference on how ICD-10 mental disorder codes are structured, see our ICD-10 code reference for other diagnostic categories.

The F06 category also includes F06.0 (psychotic disorder with hallucinations due to physiological condition) and F06.1 (catatonic disorder due to physiological condition). These are distinct presentations and should not be confused with the mood disorder subcategory. Each code requires the same two-part documentation structure: underlying condition first, specified mental disorder second.

Expert Picks

Need a structured psychiatric documentation framework? Psychiatric Evaluation Template provides a step-by-step clinical structure for capturing causality, mood features, and diagnostic criteria in a single assessment.

Looking for mental health practice management tools? Mental Health EMR covers the workflows, documentation standards, and billing integrations mental health practices need to manage complex diagnostic codes.

Managing psychology caseloads with complex co-occurring conditions? Psychology Practice Software outlines how technology supports accurate clinical documentation and streamlined billing for psychology practices.

Conclusion

Secondary mood disorders are common in patients with complex medical histories, but coding them correctly requires more than recognising depression is present. ICD-10 Code F06.31 demands explicit clinician-documented causality, correct sequencing, and specific feature documentation – three requirements that generate the majority of denials for this code family.

Pabau’s claims management software helps mental health and psychiatric practices build documentation workflows that capture the causality statements, sequencing logic, and feature specificity F06.31 requires before claims reach the payer. To see how Pabau supports complex diagnostic coding in practice, book a demo.

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